A metallic shell of a spark plug is provided with a center electrode and a ground electrode which are positioned so as to face each other having a space, what is called a spark gap, therebetween. Recently, in order to further improve fuel consumption, the ground electrode is joined to the metallic shell in an optimal igniting position when the spark plug is mounted on an internal combustion engine with a specified torque. Notably, the “optimal igniting position” means a location where a growth of flame kernel generated by spark discharge in the spark gap is not interfered by airflow of an air-fuel mixture in a combustion chamber. Henceforth, it is called the optimal igniting position in this specification.
Generally, in order to check a joint position of a ground electrode, a female thread jig is employed. The female thread jig assumes the same shape as a female thread of a cylinder head of an internal combustion engine for mounting a spark plug. In such a female thread jig, an allowable angle range of the ground electrode when the metallic shell is engaged with a female thread is indicated. The allowable angle range means an allowable deviation (angle) range of the ground electrode whose position deviates from the optimal igniting position. A person who checks the deviation of the ground electrode engages the metallic shell with the female thread jig with the same torque as that used at the time of mounting a spark plug on a cylinder head. Then, the person conducts a visual inspection to check whether or not the location of the ground electrode is in the allowable angle range and then judges the quality of a product (metallic shell).